The effect of changing slab thickness on fracture propagation

ABSTRACT: Many avalanches are triggered from shallower parts of a slope. Past research demonstrates that initiating fractures in such areas is easier than in deeper places where the applied stresses must penetrate through more overlying snow before affecting the weak layer. However, to our knowledge there is no work on whether fractures more effectively propagate from deeper to shallower areas, or from shallower to deeper areas. During the 2006/07 and 2007/08 winters, we looked at fracture propagation using standard Extended Column Tests, modified Extended Column Tests with column widths of 200 and 300 cm, and Propagation Saw Tests. We tested fracture propagation on slopes with highly variable weak layer depth and with reshaped slab thickness. Our results suggest that fractures are more likely to propagate further when traveling from under thin to thick slabs than in the opposite direction. We support our test results with four case studies. In these cases, slopes only partially released with big explosives applied where the weak layer was deeper, but then avalanched entirely a few days later when tested with small loads where the weak layer was shallower. Thus, shallower areas of the slab may be both the easiest place to initiate a fracture and also the best place from which to propagate a fracture. These results have broad implications for backcountry travel, avalanche avoidance, and avalanche control work